mlxsw: reg: Add Router Interface Table Register

Add the Router Interface Table Register (RITR), which allows us to
create and configure router interfaces (RIFs).

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>